Cellulosic and lignocellulosic feedstocks and wastes, such as agricultural residues, wood, forestry wastes, sludge from paper manufacture, and municipal and industrial solid wastes, provide a potentially large renewable feedstock for the production of chemicals, plastics, fuels and feeds. Cellulosic and lignocellulosic feedstocks and wastes, composed of cellulose, hemicellulose, pectins and of lignin are generally treated by a variety of chemical, mechanical and enzymatic means to release primarily hexose and pentose sugars, which can then be fermented to useful products.
Pretreatment methods are often used to make the polysaccharides of lignocellulosic biomass more readily accessible to cellulolytic enzymes. One of the major impediments to cellulolytic enzyme digest is the presence of lignin, a barrier that limits the access of the enzymes to their substrates, and a surface to which the enzymes bind non-productively. Because of the significant costs associated with enzymatic saccharification, it is desirable to minimize the enzyme loading by either inactivation of the lignin to enzyme adsorption or its outright extraction.
Another challenge is the inaccessibility of the cellulose to enzymatic hydrolysis either because of its protection by hemicellulose and lignin or by its crystallinity. Pretreatment methods that attempt to overcome these challenges include: steam explosion, hot water, dilute acid, ammonia fiber explosion, alkaline hydrolysis (including ammonia recycled percolation), oxidative delignification and organosolv.
Organosolv methods, as previously practiced for the treatment of lignocellulose biomass, for either the production of pulp or for biofuels applications, while generally successful in lignin removal, have suffered from poor sugar recoveries, particularly of xylose. For example, the use of slightly acidic ethanol-water mixtures (e.g., EtOH 42 weight %) at elevated temperature to remove lignin from lignocellulosic biomass (Kleinert, T. N., Tappi 57: 99-102, 1974) resulted in substantial loss of carbohydrate. Dilute acid hydrolysis at 95° C. followed by organic solvent extraction and enzymatic saccharification (Lee, Y—H. et al., Biotech. Bioeng., 29: 572-581, 1987) resulted in substantial loss of hemicellulose during hydrolysis, additional carbohydrate loss upon organic solvent extraction and poor yield (˜50% of total carbohydrate) upon enzymatic saccharification of residue.
Treatment of biomass with gaseous water and methylamine followed by extraction with organic solvent and then extraction with water, required three steps and resulted in a substantial loss of carbohydrate (Siegfried, P. and Götz, R., Chem. Eng. Technol., 15: 213-217, 1992). Treatment with polyamines or ethylamine in water-aliphatic alcohol mixtures plus catalyst at elevated temperature required high liquid/solids ratio and low concentrations of alcohol led to poor sugar recovery, particularly of xylan (U.S. Pat. No. 4,597,830A). Thioglycolate in aqueous alkaline solution used to treat lignocellulosic biomass at elevated temperature, followed by a hot water wash required use of alkali-metal or alkaline-earth hydroxides. This method requires the costly disposal of inorganic ions, high weight % thioglycolate, and use of large volumes of water (U.S. Pat. No. 3,490,993). Treatment with organic solvent-water mixtures in the presence of sulfide/bisulfide at elevated temperatures required a high solvent/solids ratio and elevated sulfur content and resulted in a substantial loss of carbohydrate, (U.S. Pat. No. 4,329,200A).
The use of aqueous organic solvent containing high concentrations of ammonia at elevated temperatures to treat lignocellulosic biomass (Park J.-K. and Phillips, J. A., Chem. Eng. Comm., 65: 187-205, 1988) required the use of a high liquid to solids ratio in pretreatment and resulted in substantial loss of hemicellulose and poor enzymatic saccharification of cellulose.
Additional shortcomings of previously applied methods include, separate hexose and pentose streams (e.g., dilute acid), inadequate lignin extraction or lack of separation of extracted lignin from polysaccharide, particularly in those feedstocks with high lignin content (e.g., sugar cane bagasse, softwoods), disposal of waste products (e.g., salts formed upon neutralization of acid or base), and poor recoveries of carbohydrate due to breakdown or loss in wash steps. Other problems include the high cost of energy, capital equipment, and pretreatment catalyst recovery, and incompatibility with saccharification enzymes.
One of the major challenges of biomass pretreatment is to maximize the extraction or chemical neutralization (with respect to non-productive binding of cellulolytic enzymes) of the lignin while minimizing the loss of carbohydrate (cellulose plus hemicellulose) via low-cost efficient processes. The higher the selectivity, the higher the overall yield of monomeric sugars following combined pretreatment and enzymatic saccharification.
There is therefore a need to develop a single step process using substantially lower liquid to solid ratio in pretreatment and recyclable base without substantial loss of hemicellulose and poor enzymatic saccharification of cellulose. The current disclosure addresses this need. In this disclosure, low to moderate concentrations of ammonia in the presence of one or more nucleophiles is used for an organic solvent solution-mediated fragmentation and selective extraction of lignin at elevated temperatures under alkaline conditions. This cost-effective process maintains the hemicellulose content of the biomass while selectively removing lignin and produces carbohydrate-enriched biomass that is highly susceptible to enzymatic saccharification. The pretreated biomass produced by the processes described herein results in very high yields of fermentable sugars (glucose, as well as xylose) following saccharification and in turn, high yields of target products (e.g., value-added chemicals and fuels) after fermentation. Surprisingly, use of low to moderate concentrations of ammonia and in the presence of one or more nucleophile resulted in significantly improved lignin fragmentation and extraction and high carbohydrate retention, particularly with respect to hemicellulose.